My trials, tribulation and progress in designing and building a handcycle, at home, out of carbon fiber.

Thursday, February 9, 2012

Self-Centering Steering - Finally a good design

Steering is an important consideration. When I come to a stop, I may need to put both hands down in order to balance and push the handcycle with my hands. Of course, if my hands are taken off the steering, the steering could flop to one side or another. Therefore, I need a self-centering steering -- no hands involved.

I believe I have a design for it. It will require two torsion springs (a mouse trap has a torsion spring). Why two springs? A torsion spring only works in one direction. It should not go back past the zero spot. That is, a torsion spring should not be torque in the opposite direction from which it was built for. Yes, it will go back to "zero", but over time the spring may break if used in the wrong direction. A spring can be a left or a right-hand spring.

The design that I have come up with requires two torsion springs -- one for right-hand turns and one for left-hand turns. The opposite spring would stay in the zero position throughout the other's torsion.

The springs can be built from piano wire or stainless (or other material), These would use stainless.

The steering needs very few degrees of movement. I still need to determine the amount. But I suspect it is less than ten degrees.

The springs need a mandrel. In this case, the perfect mandrel is the steerer tube. The steerer tube will be extended toward the wheel. Of course the benefit of this is that the center of the turn is the center of the steerer tube. One end of each spring will fit into a hole in the bottom of the fork. As the fork turns around the steerer tube, the spring end would move accordingly around its center -- the center of the mandrel (the steerer tube). The other ends of the springs would be allowed to move in one direction only through slots in the base of the carbon fiber. (See picture below.) The right spring for the right turn would add torsion while the left spring stays in neutral (and vice versa).. Perfect...

The diameter of the stainless wire used to build the spring will impart the number of foot-lbs per degree of swing. It is additive in that the further the rotation of the spring end, the greater the force. This is exactly what I want.

2 comments:

I like this. Using your hands for balance when you stop. I had all kinds of kickstand-like contraptions in my head, but I think this makes more sense -- with the right gloves.

One thing, will the two springs wear evenly, or are you going to end up leaning/fighting in one direction or another. You'll need to compensate with a tensioner.

Another thought (just my 2 cents) would be a triangular steering shaft (maybe more of a triangle/hexagon) and then a tensioner/spring that would hold the "pointed" end into the housing. When the wheel turns, the shape would naturally lift out of the housing a bit stressing the tensioner, which would tend to pull it back to center evenly in both directions. Your thoughts?

The torsion springs must not go "negative". That is one will never fight the other in this design. Only one spring works at a time. The other spring (the one without force on it) completely turns since that end is free in the slot. Each slot (one for each spring) allows one spring end to move freely from neutral to whatever length the other spring is working across. Therefore, both springs will not fight the other and there will not be excessive wear. More importantly, neither spring will go "negative" which would result in premature failure.

Relative to your second point -- I have another design that I hope to build this weekend. It will have two compression springs. Like the above design, each will work independently of the other -- and never fight each other. I will have a pipe elbow (if you will) with closed ends. There will be two spring in the pipe elbow. Both springs will be neutral -- as you buy it. There will be an open slot in the elbow for which a "finger" (again, if you will) will fit into the elbow between both springs. When the finger moves in one direction, that particular spring will compress. The other spring will remain neutral. As the spring will push the finger back to neutral with the bike going in a straight line at that point. As the finger passes through neural (if I am steering in the other direction), the other spring will take over. This is even a simpler design.

What I don't know is if the compression spring will work well when the springs are put within a container (the closed-end plumbing elbow) that has turning radius. But it will be easy to build a test out of a small plumbing plastic elbow with two springs. I can easily cut a slot in it with, say, a 20 degree turn (10 degrees in each direction). It will be a fun test anyway and I won't have to build it into the handcycle initially.